Xenobiotic transporters play a critical role in drug disposition and response. Among recognized examples, the organic cation transporter 1 (OCT1) has been demonstrated by us as a key determinant of the therapeutic response to metformin, one of top ten prescription drugs in the United States. Despite their clinical significance, many xenobiotic transporters, particularly those uptake transporters including OCT1, are poorly characterized in their regulation. The recently identified ischemia/reperfusion-inducible protein (IRIP) regulates a variety of transporters in vitro mediating membrane transport of endogenous metabolites and xenobiotics. Therefore, IRIP may represent a novel regulatory mechanism for transporter activity with a substantial effect on clinical drug therapy, and studies are highly warranted to characterize the, as of yet unknown, IRIP function in the body. Our exciting preliminary results indicate that hepatic overexpression of IRIP is associated with a strikingly reduced hepatic accumulation of metformin in the mice administrated with the drug. We also observed altered metformin accumulation and IRIP expression in the livers of obese mice and those received the endotoxin lipopolysaccharide (LPS). Using gene targeting, we have generated a knockin mouse strain that can be used to generate global and conditional IRIP knockout mice. These knockout mice, along with our established cell models and gene delivery techniques, provide a powerful system to test our central hypothesis that IRIP expression affects the disposition and response of clinically important drugs which are substrates of the xenobiotic transporters regulated by IRIP. We propose three specific research aims. In Aim 1, by using genetic manipulation and model compounds in cell models, we will delineate the mechanism of transporter regulation by IRIP at the protein activity, cellular and molecular levels. In Aim 2, we will determine how liver- specific and ubiquitous alteration in IRIP expression affects transporter function and metformin disposition. Hepatic conditional IRIP knockout (mIRIP?hep) mice, hepatic IRIP overexpression (mIRIP+hep), global IRIP knockout (mIRIP-/-), and their corresponding control mice will be employed to perform the studies in Aim 2. In addition, as transporter activities and drug disposition are significantly changed during pathophysiological conditions, in Aim 3 we will use the mouse and in vitro cellular models to define the role of IRIP in regulation of drug disposition during fatty liver disease, lipopolysaccharide-induced endotoxemia and ischemia/ reperfusion injury. The proposed research will characterize the first function of mammalian IRIP in the body. The results will significantly aid in our understanding of xenobiotic transporter regulation and provide a target for improvement of drug efficacy and avoidance of drug side effects for patients.